1. Field of the Invention
The invention relates to a method of bonding tungsten-titanium sputter targets to titanium plates and to the target assemblies produced thereby. The bond between the tungsten-titanium targets and the titanium plates exhibit a high bond strength and a capability of withstanding high sputtering power levels.
2. Description of the Prior Art
Sputtering as a means to deposit thin films of a desired material on a substrate has become important in the manufacture of semi-conductor devices, such as integrated circuits. In a sputtering system, material to be deposited on a substrate is removed from a sputter target by bombardment of the target with ions.
In addition to the ion bombardment causing atoms or molecules of the target material to be ejected from the target, the process also causes thermal energy to be imparted to the target. In order to prevent the target from overheating, the target is typically mounted to a backing plate to facilitate cooling, and cooling water is circulated in contact with the backing plate such that heat energy is transferred from the target to the cooled backing plate. Thus, it is important that a good thermal bond exists between the target and the backing plate.
In one version of a sputter target assembly, the sputter target is physically soldered to the backing plate. For example, a tungsten-titanium sputter target may be soldered to a copper or austenitic (non-magnetic) stainless steel backing plate using indium, tin-indium or tin-silver alloy. Alternatively, a sputtered or electroplated metallic film may be applied to either or both the target and backing plate to enhance the wettability of the bond surfaces being joined by the solder connection, and in some cases a silver filled epoxy is used as the bonding agent.
In this sputtering design, the bonding surfaces between the target and the backing plate are planar and the backing plate further includes an annular ring extending downwardly from the planar surface wherein cooling of the target is accomplished by circulating water into a chamber formed within the annular ring and against the lower surface of the planar backing plate. In the past, this particular design of sputter target assembly has proven to have several drawbacks.
Since the target and backing plate are formed of different materials, they have different rates of thermal expansion. As a result, subsequent to heating of the target and backing plate to form the solder bond therebetween, the cooling of the assembly results in the target material being bowed wherein the target assumes a convex shape due to the target having a lower rate of thermal expansion than the backing plate. Thus, a flattening operation is subsequently required to bring the target back to its original planar shape. This flattening operation induces a strain on the bond area such that the solder must be sufficiently forgiving to accommodate the strain induced by this thermal distortion.
In addition, if the sputter target is operated at a high sputtering power level, there is the danger of the target becoming overheated to the point of causing the solder joint between the target and the backing plate to melt, thus causing failures in the sputter target equipment, resulting in needless downtime of the equipment.
In some target assemblies the degree of bowing of the target assembly is minimized by incorporating a backing plate made of titanium which has a thermal expansion rate more closely matched to that of the tungsten-titanium target. However, it is extremely difficult to produce a reliable solder bond with the titanium plate because of the non-wetting characteristics of titanium. Further, if a suitable bond is formed with one of the solders having a relatively low melting point, the utility of the assembly is limited to the lower operating temperatures for the target, such that the use of the target assembly is limited.
Accordingly, there is a need for a sputter target assembly wherein the thermal expansion rates of the target and an associated backing plate are closely matched. In addition, there is a need for such a target assembly wherein a durable bond is formed between a tungsten-titanium target and a titanium backing plate.